Carbon dioxide is used in a number of industrial and domestic applications, many of which require the carbon dioxide to be free from various impurities. Unfortunately, carbon dioxide obtained from natural sources such as gas wells, chemical processes, fermentation processes or produced in industry, particularly carbon dioxide produced by the combustion of hydrocarbon products, often contains impurity levels of sulfur compounds such as carbonyl sulfide (COS) and hydrogen sulfide (H2S). When the carbon dioxide is intended for use in an application that requires the carbon dioxide to be of high purity, such as in the manufacture and cleaning of foodstuffs and beverage carbonation, medical products and electronic devices, the sulfur compounds contained in the gas stream must be removed to very low levels prior to use. The level of impurity removal required varies according to the application of carbon dioxide. For example, for beverage application the total sulfur level in carbon dioxide (CO2) ideally should be below 0.1 ppm. Removal to similar levels is required for electronic cleaning applications.
Various methods for removing sulfur compounds and hydrocarbon impurities from gases such as carbon dioxide are known. For example, U.S. Pat. No. 4,332,781, issued to Lieder et al., discloses the removal of COS and H2S from a gas stream by first removing the H2S from the hydrocarbon gas stream by contacting the gas stream with an aqueous solution of a regenerable oxidizing reactant, which may be a polyvalent metallic ion, such as iron, vanadium, copper, etc., to produce a COS-containing gas stream and an aqueous mixture containing sulfur and reduced reactant. The COS in the gas stream is subsequently hydrolyzed to CO2 and H2S by contacting the gas stream with water and a suitable hydrolysis catalyst, such as nickel, platinum, palladium, etc., after which the H2S and, if desired, the CO2 are removed. This step can be accomplished by the earlier described H2S removal step or by absorption. The above-described process involves the use of cumbersome and costly equipment and liquid-based systems which require considerable attention and may result in the introduction of undesirable compounds, such as water vapor, into the carbon dioxide product.
U.S. Pat. Nos. 5,858,068 and 6,099,619 describe the use of a silver exchanged faujasite and an MFI-type molecular sieve for the removal of sulfur, oxygen and other impurities from carbon dioxide intended for food-related use. U.S. Pat. No. 5,674,463 describes the use of hydrolysis and reaction with metal oxides such as ferric oxide for the removal of carbonyl sulfide and hydrogen sulfide impurities from carbon dioxide.
It is known to directly remove sulfur compounds, such as H2S from a gas stream by contacting the gas stream with metal oxides, such as copper oxide, zinc oxide or mixtures of these. It is also known to remove sulfur impurities such as COS by first hydrolyzing COS to H2S over a hydrolysis catalyst and then removing H2S by reaction with metal oxides. Removal of H2S by reaction with metal oxides can become expensive, since the catalyst is non-regenerable and expensive, when impurities such as COS and H2S are present in more than trace amounts. Lower cost materials for the removal of COS and H2S and other sulfur impurities such as mercaptans and dimethyl sulfide are desired to reduce CO2 purification cost.
Since many end users of carbon dioxide require the carbon dioxide they use to be substantially free of sulfur compounds, and because natural sources of carbon dioxide and industrially manufactured carbon dioxide often contain sulfur compounds, economic and efficient methods for effecting substantially complete removal of sulfur compounds from carbon dioxide gas streams, without concomitantly introducing other impurities into the carbon dioxide, are continuously sought. The present invention provides a simple and efficient methods for achieving these objectives.